Digital Subscriber Line (DSL) technology was initially deployed to provide data-only service as a alternative for slower-speed dialup modems. Incumbent Local Exchange Carriers (ILECs), Competitive Local Exchange Carriers (CLECs), and other telecommunication providers have begun to explore offering Voice over DSL (VoDSL) service to deliver integrated voice and data services.
A typical VoDSL system includes a gateway that interfaces with a telecommunication switch, such as a Class 5 switch on a Public Switched Telephone Network (PSTN), and an Integrated Access Device (IAD) at the customer premises. The gateway sends and receives digital signals in a Time-Division Multiplexed (TDM) format to and from the telecommunication switch and communicates with the IAD using Asynchronous Transfer Mode (ATM) cells. The IAD sends and receives telecommunications information, such as analog voice signals, to and from customer equipment, such as a telephone on a subscriber line.
Specifically, when receiving voice signals from a telephone, the IAD converts the voice signals into ATM cells and transmits the ATM cells to the gateway. Typically, the VoDSL system uses the ATM adaptation layer protocol known as AAL2 for the ATM cells, with the ATM cells transported to and from each IAD on an ATM Permanent Virtual Circuit (PVC). The gateway converts the ATM cells from the IAD into TDM signals and transmits the TDM signals to the telecommunication switch. Specifically, the TDM signals are typically sent to GR-303 Digital Signal (e.g., DS-0) channels in the telecommunication switch.
Gateways utilize cross-connect functionality to provide distribution paths between the PVCs and the DS-0s. Since the ATM PVCs are typically statically configured by a craft person while DS-0s are typically dynamically assigned by the Class 5 switch, the gateway must be able to dynamically connect any static AAL2 channel to any DS-0 within any GR-303 group.
A typical gateway utilizes a central Segmentation And Reassembly (SAR) module and a TDM switch to provide the cross-connect function. The central SAR module converts the ATM cells into TDM signals. The TDM switch cross-connects the TDM signals from the central SAR module to the appropriate DS-0 of the telecommunication switch via TDM ports. For communications from the telecommunication switch back to the customer equipment, the VoDSL system uses generally the reverse of the operations described above.
Using ATM cells rather than conventional TDM protocols allows for more effective and efficient use of bandwidth between the customer premises and the telecommunication switch. A disadvantage associated with the typical VoDSL system, however, is that it requires a TDM switch to cross-connect the central SAR with the TDM ports. That requirement increases the cost of the system.
Another disadvantage relates to the capacity of conventional central SARs. Providers of telecommunication services desire a VoDSL gateway with the capacity to handle over 3000 simultaneous calls and 10:1 over-subscription. This approximates the bandwidth of two Optical Carrier-3 (OC-3) channels, or approximately 50,000 connections. Current AAL2 central SARs, however, only support 1023 simultaneous calls and connections. Conventional VoDSL systems are therefore incapable of providing the desired capacity.